Laser Actuation of Cantilevers for Picometre Amplitude Dynamic

OPEN Laser Actuation of Cantilevers for

SUBJECT AREAS: Picometre Amplitude Dynamic Force CHARACTERIZATION AND ANALYTICAL Microscopy TECHNIQUES POLYMERS Drew R. Evans1,2, Ponlawat Tayati1, Hongjie An1, Ping Koy Lam3, Vincent S. J. Craig1 & Tim J. Senden1 CHEMICAL PHYSICS

1Department of Applied Mathematics, Research School of Physics and Engineering, The Australian National University, Canberra, ACT, 0200, Australia, 2Thin Film Coatings Group, Mawson Institute, University of South Australia, Mawson Lakes, SA 5095, Received 3 Australia, Department of Quantum Science, Research School of Physics and Engineering, The Australian National University, 24 March 2014 Canberra, ACT, 0200, Australia. Accepted 5 June 2014 As nanoscale and molecular devices become reality, the ability to probe materials on these scales is Published increasing in importance. To address this, we have developed a dynamic force microscopy technique where 4 July 2014 the flexure of the microcantilever is excited using an intensity modulated laser beam to achieve modulation on the picoscale. The flexure arises from thermally induced bending through differential expansion and the conservation of momentum when the photons are reflected and absorbed by the cantilever. In this study, we investigated the photothermal and photon pressure responses of monolithic and layered cantilevers using a Correspondence and modulated laser in air and immersed in water. The developed photon actuation technique is applied to the requests for materials stretching of single polymer chains. should be addressed to T.J.S. (tim.senden@ he Atomic Force Microscope (AFM) is widely used for high resolution imaging of surfaces, performing force anu.edu.au) or analysis, surface characterization1,2, and manipulating molecular systems ranging from DNA3 and motor V.S.J.C. (vince.craig@ proteins to classical polymer chains4–7. The theoretical understanding of performing work on molecular T 8–10 anu.edu.au) systems is well established , however, the experimental ability to perform nano-mechanical work on molecular systems has been problematic. The difficulty lies in the detection of changes in low energy regimes where the work performed on a system is comparable in magnitude to that of the work done by thermal fluctuations, due to Brownian motion. In this sense, the accuracy of AFM measurement at the molecular scale is said to be thermal fluctuation limited11–13. Minimizing the effect of thermal noise can be achieved either by averaging many measurements (although this has some inherent issues14) or by modulating the sensor such that the contribution of thermal noise at the modulation frequency is small in comparison to the total measureable signal. The latter principle is at the heart of dynamic force microscopy. A key feature of dynamic AFM is the method used to produce the cantilever excitation. Usually, a piezo-electric actuator or magnetic particle are used to generate acoustic and magnetic excitation15,16, with the excitation method influencing the mode of vibration17. Recently it has been argued that piezoacoustic excitation of cantilevers can preclude accurate interpretation of data18. Optical excitation of AFM cantilevers has been achieved by modulating the intensity of a laser impinging on the cantilever both in air19–21 and in liquid22,23. This method produces a frequency response unaffected by spurious contributions of noise from mechanical coupling through liquids, thus providing an opportunity to explore details of hydrodynamic and fluid systems. In this study, we use a modulated blue laser (wavelength: 405 nm) to excite an AFM cantilever. There are two ways to induce cantilever flexure via a laser: thermal heating, which leads to differential expansion and photon pressure. The latter was theoretically predicted by James Clerk Maxwell in the 1860’s24, whereby light (or indeed any radiation) exerts a small force on the surface it impinges. The relationship between the power of the impinging radiation and the exerted force is given by Equation 125. 2P zP F ~ R A ð1Þ photon c

Where FPhoton is the photon pressure force (N), PR is the fraction of photon power reflected (W) from the surface, PA is the fraction of absorbed photon power (W) and c is the speed of light (m/s). It must be noted here that the functional form of Equation 1 for a cantilever would require knowledge of reflectivity and absorptivity of the cantilever, which varies with the laser wavelength and optical geometry (angle of incidence).

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Methods CSC-38 are rectangular silicon cantilevers with an aluminium coating (Mikromasch). When a load is applied to a cantilever, it will deflect until a force balance is reached Water used was purified using a MilliQ Gradient system. Silicon substrates were between the applied load and restoring force of the cantilever. Using the framework cleaned using a RF frequency water plasma (10 W 45 s). PNIPAM (MW 5 5035, 27 outlined in26, the photon pressure induced force per unit length of the cantilever, can Polydispersity Index (PI) 5 2.1) was synthesised according to Zhou et al. , preci- 13 be related to the measured deflection of the cantilever under a distributed load by pitated twice from acetone/n-hexane and confirmed with C-NMR. Equation 2, (2P zP )(a2{3az3) Results and Discussion dopt ~ R A ð2Þ 3cka(a{2)2 The frequency response spectrum of a laser modulated AFM cantilever is complex. The photothermal amplitude is largest at low where, dopt is the end load calibrated measured deflection from the optical lever (m) [ie the standard detection system used on many commercial instruments]; a is the modulation frequencies as the cantilever is able to cool and heat on normalised measurement position (a 5 x/L where x is the distance of measurement the time scale of the driving frequency and it oscillates between laser spot from the base of the cantilever and L is the length of the cantilever); and k is deflection positions corresponding to the warm and cool states. As the spring constant of the cantilever (N/m). We use this equation to calculate the the frequency is increased, the time available for cooling and heating deflection measured using the optical lever technique for a given total laser power, and compare it to the observed deflection. For static measurements, dopt is a deflection is decreased and consequently the amplitude of oscillation decreases. of the cantilever caused by continuous laser illumination. To perform dynamic At sufficiently high modulation frequencies the cantilever effectively measurements, the cantilever is modulated and the response of the cantilever is remains at a constant temperature and oscillation due to the photo- observed at the actuated frequency. Given we are using a dynamic method, the thermal effect becomes negligible. In comparison the contribution amplitude response of the cantilever due to the photon pressure can be derived from Equation 2 as, due to photon pressure is not expected to show any frequency dependence. As the excitation frequency increases other bending 1 (2P zP )(a2{3az3) ARMS~ pffiffiffi R A ð3Þ modes can result in broadening of the resonance peak. Evidently { 2 2 2 3cka(a 2) the contribution due to photon pressure, will be most evident at The amplitude response due to the photon pressure can be calculated for a cantilever frequencies sufficiently large that the photothermal oscillation is with known spring constant, measurement position, and the reflected and absorbed reduced but below frequencies where the resonance of the cantilever photon power. Here we use a modulated laser beam, in the arrangement depicted in is evident. We call this frequency window the photon pressure Figure 1, to fully illuminate the underside of an AFM cantilever, causing it to oscillate. When light impinges on the AFM cantilever a portion of the light is absorbed by the regime. cantilever, resulting in heating, the remainder is reflected. By conservation of The frequency response spectra of a Multi75-Al cantilever with momentum, there is a net transfer of momentum from the photons to the cantilever, and without laser excitation is shown in Figure 2. Without the addi- causing the cantilever to deflect in response. To demonstrate this technique, an AFM tional excitation, the (thermally driven) frequency spectrum shows (Research MFP-3D, Santa Barbara, CA) was modified to incorporate a modulated laser unit (Coherent Compass Laser, l 5 405 nm) where the modulation signal is the fundamental resonance frequency of a cantilever to be 63.6 kHz produced from an arbitrary function generator (Hewlett Packard/Agilent with quality factor, Q 5 155. The laser actuated spectrum yields HP33120A). The modulated laser beam was coupled to the AFM via an inverted approximately the same fundamental resonance frequency, however microscope (Nikon Eclipse TE 2000-U). The dynamic deflection amplitude was the peak is broadened. It is worth noting that the laser actuated adjusted by varying the total incident laser power using neutral density (ND) filters. The power output of the laser was measured using a power meter (Field master GS, spectrum does not show any additional spurious oscillation modes Coherent). Cantilever deflection was detected using the inbuilt conventional optics of which would otherwise appear as artefacts in the spectrum, implying the Asylum AFM. The oscillatory component of the deflection signal was isolated no other vibrational mode is excited, only the first flexural vibrational using a lock-in-amplifier (Stanford Research Systems, SR 830 DSP), which was locked mode. The lower limit to the amplitude measurement is illustrated by to the modulation frequency, as defined by the reference signal of the function generator. obtaining a frequency response spectrum of the cantilever immersed A range of AFM cantilevers from different suppliers were used in this investigation in water. By immersing the cantilever in water, the resonance fre- with the following designations. Multi 75 G and Multi 75 are rectangular silicon quency is reduced (effective mass of the cantilever increases), the Q is cantilevers without coating and coated with aluminium respectively (Budget reduced (increased damping) and it becomes possible to probe fre- Sensors). Contact G are rectangular silicon cantilevers without a coating (Budget Sensors) CSG 10 are rectangular single crystal silicon cantilevers with a gold coating quencies above resonance without exceeding the upper frequency (NT-MDT). SNL-10 are V shaped silicon nitride cantilevers with a gold coating limit of the lock-in amplifier used in our measurement (102 kHz). (Bruker). PNP-DB-00x are rectangular silicon nitride cantilevers without a coating. An example of this situation is shown in Figure 2C, where the mea-

Figure 1 | A schematic of the experimental configuration employed. The modulated laser beam is coupled to the AFM via an inverted optical microscope. In our arrangement the beam is not focussed to a point but rather floods the whole of the lower side of the cantilever. The influence of focussed beams on cantilever bending has been studied previously22,23. The deflection of the cantilever is measured using the conventional optical lever technique, reflecting from the upper side of the cantilever. The oscillatory response of the cantilever is isolated using a lock-in amplifier.

SCIENTIFIC REPORTS | 4 : 5567 | DOI: 10.1038/srep05567 2 www.nature.com/scientificreports

Figure 2 | Amplitude versus frequency response of an aluminium coated silicon cantilever (Multi 75-Al). The thermally driven response spectrum (modulated laser is power off) is shown in Panel A. Laser actuated spectra obtained in air (Panel B) and water (Panel C) were obtained when the power of the impinging modulated laser was 7.5 mW. sured RMS amplitude is several picometers at frequencies above the minium-coated cantilever . uncoated cantilever, for both silicon resonance for the first mode of vibration. and silicon nitride cantilevers. Both uncoated and metal-coated cantilevers could be excited in air Considering the laser always strikes on the lower surface of all and in water at room temperature (Figure 3 and Figure 4). The cantilevers examined, the reflection coefficient, was calculated relative magnitude of the amplitude response for different types of according to Fresnel equations. The incident angle is typically cantilever actuated in air, with the same power level were quite ,10u due to the AFM instrument design, where a mounted can- different. The measured amplitude response from laser actuated tilever is tilted relative to horizontal. Given the wavelength of light cantilevers followed the trend: gold-coated cantilever . alu- employed, 405 nm, the reflection coefficient for silicon and silicon

SCIENTIFIC REPORTS | 4 : 5567 | DOI: 10.1038/srep05567 3 www.nature.com/scientificreports

Figure 3 | Amplitude response of a laser actuated gold coated silicon cantilever (CSG-10, 10 mW) (A) in air (B) in water; and an uncoated cantilever (Multi75-G, 7.5 mW) (C) in air and (D) in water. nitride is 0.475 (refractive index 5.4428) and 0.125 (refractive index Therefore the transmission of light through the silicon cantilever can 2.0729) respectively. At thicknesses above 100 nm, silicon layers are be neglected here30. The absorption coefficient of a typical silicon opaque to light radiation with wavelengths of 250 nm to 500 nm. cantilever is 0.525. Since the absorption coefficient for silicon nitride

Figure 4 | Amplitude response of a laser actuated gold coated silicon nitride cantilever (SNL-10, 16 mW) (A) in air (B) in water; and an uncoated cantilever (PNP-DB, 10 mW) (C) in air and (D) in water.

SCIENTIFIC REPORTS | 4 : 5567 | DOI: 10.1038/srep05567 4 www.nature.com/scientificreports is 105 m2131, the absorbed light intensity is 0.051 times the input the top of the cantilever is 16.8 mK for a total incident power of intensity upon traversing a silicon nitride cantilever of 0.6 mm thick- 10 mW and the resulting deflection at the end of the cantilever is ness, and the light reflected at the other side of the cantilever will 0.37 pm. Due to the low absorption coefficient of silicon nitride, c 5 5 21 2cz contribute to the photon pressure through a second reflection. The 10 m , the absorbed power is PA 5 Pin(1 2 0.125)(1 2 e ) 5 intensity of the second reflection normalised by the incident radi- 0.51 mW for a cantilever of 0.6 mm thickness and a total incident ation is 0.104. For simplicity we ignore subsequent reflections. The power of 10 mW. In comparison the calculated temperature differ- amplitude response from the photon pressure can then be calculated ence from the bottom to the top of a 0.6 mm thick uncoated silicon from Equation 3, giving an estimate of the amplitude from photon nitride cantilever is only 1.2 mK (PNP-DB-00x, l 5 32 W K21 m21, pressure (Table 1). c 5 3 3 1026 K21, l 5 200 mm, w 5 40 mm, and h 5 0.6 mm). In the photon pressure regime, the measured amplitudes of coated Assuming an emissivity of 1, the thermal expansion causes a deflec- cantilevers are higher by 3 orders of magnitude than the estimated tion at the end of the cantilever of 0.12 pm. Whilst the manufacturers photon pressure amplitudes. For uncoated silicon cantilevers, the state that these cantilevers have no coating the silicon nitride will differences are 2 orders of magnitude. The amplitude response of have a native oxide layer (,1 nm, thermal conductivity l 5 1.3 W uncoated silicon nitride cantilevers is closer to the estimated photon K21 m21, c 5 5 3 1027 K21, young’s moduli E 5 70 GPa), therefore pressure amplitude, but still remains approximately 20 times higher. the cantilever will bend due to differential thermal expansion of this For coated cantilevers, the cantilever will bend due to difference in layer and the substrate. The bending due to differences in thermal the thermal expansion coefficient of the coating and the substrate. expansion coefficients can be calculated using Equation (5). The The deflection can be calculated at a constant steady-state deflection calculated deflection of a silicon nitride cantilever due to thermal for a uniformly distributed continuous laser using Equation 432. expansion with an oxide layer of thickness 1 nm at 10 kHz modulation of laser (power 5 10 mW) is 0.1 pm. However, our data shows 5 t zt l3 z ~{ (c {c ) 1 2 P ð4Þ deflections several times the sum of the calculated thermal and 0 1 2 2 z 4 t2 K (l1t1 l2t2)W photon pressure amplitudes. We note the silicon nitride cantilever Where, z is the vertical deflection of cantilever, c thermal expansion is affixed to a substrate of Pyrex glass, having a different thermal coefficient of layers, t the thickness of the layers, l the length, and w expansion coefficient. It is possible that the stress in this joint is the width of the cantilever and P the absorbed power. The deflection influenced by temperature. This may be the origin of the spurious of a coated cantilever actuated at different frequencies was plotted in oscillation of the cantilever. Figure 5A. The response of the cantilever to heating effects can be In contrast to other modulation methods, the laser actuation tech- fitted using an exponential, and thus Equation 4 can be modified into nique has the ability to excite a cantilever at a frequency both above Equation 5 when a pulsed laser is employed. and well beyond the fundamental resonance frequency whilst main- taining a sub nanometre to nanometre oscillation amplitude. This 3 { 5 t1zt2 l {(x x0) z~{ (c {c ) P (A {Aexp t ) ð5Þ enables time-resolved measurements to be performed as well as 1 2 2 z A 0 4 t2 K (l1t1 l2t2)W probing time-dependent dynamics of molecular systems. We have applied our technique to time-dependent dynamics of t Where, is thermal relaxation time constant, and can be obtained by molecular systems. The laser actuated AFM was employed to look at fitting an exponential to the deflection versus time curve. The ther- the dynamics of polymer chain stretching between an AFM tip and a mal dynamic deflection amplitude can then be estimated (Figure 5B). silica substrate. The purpose of this example is to demonstrate the The bending of uncoated cantilevers can be induced due to a applicability of the technique. A detailed discussion of the dynamics temperature gradient induced by the absorption of light. The tem- of polymer chain extension forces is beyond the scope of this perature gradient and thermal expansion can be calculated according 34–37 33 work . to Equation 6 and 7 . In our experiment, we measured the force required to stretch poly hP (N-isopropylacrylamide) (PNIPAM) chains which were adsorbed DT~ A ð6Þ llw onto a glass slide in water (Figure 6). The tip has initially been brought into contact with the surface. Upon retraction, a polymer DTl2 chain on some occasions forms a bridge between the two surfaces. Z(l)~{c ð7Þ 2h The chain is then stretched as the tip is retracted further. As well as monitoring the conventional force-distance profile, we also moni- where l is the thermal conductivity, c thermal expansion coefficient, tored the oscillation amplitude and phase. We found the laser h the thickness of the cantilever, l its length, w its width, DT the actuation technique was very sensitive to molecular events during temperature difference from the bottom to the top, and Z(l) is the single molecule extensions. This data can be used to calculate the thermal expansion caused by the temperature difference between the interaction stiffness and damping coefficient using Equations (8) and bottom and the top. For uncoated silicon cantilevers (Multi 75 G, (9). l 5 149 W K21 m21, c 5 2.6 3 1026 K21, l 5 225 mm, w 5 28 mm, A and h 5 3 mm), the temperature difference between the bottom and k ~k ( 0 cos h{1) ð8Þ int L A

Table 1 | Measured and estimated photon pressure amplitude of different cantilevers

Measured Amplitude in the Photon Estimated Photon Pressure Amplitude Cantilever type Spring constant (N/m) Power (mW) pressure regime (pm) (pm) CSG-10 0.24 10 539 0.027 Multi 75-Al 1.21 7.5 7.9 0.004 Multi 75-G 1.81 7.5 1.33 0.003 Contact G 0.1 20 16.4 0.13 SNL-10 0.11 16 325 0.033 PNP-DB-00x 0.05 20 1.59 0.09 PNP-DB-00x 0.046 10 0.85 0.05

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Figure 5 | A) The bending of a coated cantilever (CSG-10, NTMDT) due to laser modulation with an incident power of 200 mW as a function of frequency and B) comparison of the measured and estimated thermal amplitude response for a coated cantilever (CSG-10, NTMDT), using an incident power of 10 mW.

k A and increase in the interaction stiffness and damping of the system. c~{ L 0 sin h ð9Þ At a tip-surface separation of 41.5 nm the bridging molecule is Av released and the measurements return to baseline values. where kL is the cantilever stiffness, A0 and A are the free and interaction amplitudes respectively, h is the phase (in radian) and v is the Summary angular frequency of cantilever oscillation38. We have demonstrated actuation of an AFM cantilever using a modulated laser and applied this to single molecule extensions. We v~ p 2 f ð10Þ found that the response of the cantilever is dominated by the photo- The results of an experiment where a single molecule is extended is thermal effect, rather than photon pressure. This technique enables shown in Figure 7. As the surfaces are separated a single molecule is very small cantilever oscillation amplitudes to be employed over a stretched between the surfaces. This is revealed as an attractive force wide frequency range. Armed with the ability to modulate a cantilever from sub-resonance frequencies up to and beyond the resonance frequency, time-dependent dynamics of molecular systems becomes accessible by AFM.

Figure 6 | Measurements using a cantilever (CSC-38) driven by laser modulation at 6 kHz using with a free amplitude of 1200 pm, during the extension of a single Poly(N-Isopropylacrylamide) (PNIPAM) polymer Figure 7 | The force, interaction stiffness and damping coefficient of a chain from a silicon wafer in water. (A). The force versus separation photon pressure driven AFM cantilever during the extension of a single measured in the conventional manner. (B). Amplitude of oscillation versus PNIPAM polymer chain. (A). A conventional force versus separation plot. separation. (C). The relative phase response of the cantilever to the laser (B). Interactions stiffness and (C) Interaction damping coefficient at modulation. 6 kHz using modulated laser excitation with a free amplitude of 1200 pm.

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Dynamical characterization of vibrating AFM cantilevers forced by photothermal excitation. Phys Rev B 81, How to cite this article: Evans, D.R. et al. Laser Actuation of Cantilevers for Picometre 054302 (2010). Amplitude Dynamic Force Microscopy. Sci. Rep. 4, 5567; DOI:10.1038/srep05567 (2014). 22. Bircher, B. A., Duempelmann, L., Lang, H. P., Gerber, C. & Braun, T. Photothermal excitation of microcantilevers in liquid: effect of the excitation laser This work is licensed under a Creative Commons Attribution 4.0 International position on temperature and vibrational amplitude. Micro & Nano Letters 8, License. The images or other third party material in this article are included in the 770–774; DOI:10.1049/mnl.2013.0352 (2013). article’s Creative Commons license, unless indicated otherwise in the credit line; if 23. Ramos, D., Tamayo, J., Mertens, J. & Calleja, M. Photothermal excitation of the material is not included under the Creative Commons license, users will need microcantilevers in liquids. J Appl Phys 99, 124904 (2006). to obtain permission from the license holder in order to reproduce the material. To 24. Maxwell, J. C. A treatise on electricity and magnetism. (Clarendon Press, 1873). view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

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